Air bags are inflatable bags that remain folded and out of sight in readiness for a frontal collision. During a collision, chemical reaction of a material typically containing sodium azide produces gaseous products which inflate the bag and interpose it between the driver and the steering wheel or the front seat occupant and the dashboard.
Belt tensioners are devices that tighten automobile seat belts during a crash to hold the seat occupants more securely. A tensioner has a small motor driven by pressurized gas from the chemical reaction of gas generating material like that used for inflating air bags. In both cases, chemical reaction of the gas generating material is initiated by an electrically heated squib. The crash sensor of this invention is a switch for controlling the power that energizes the squib.
In the United States automobiles having electrically initiated air bag inflators are required by law to incorporate diagnostic capability to warn the driver of a failure in the firing circuit.
A crash sensor of the type to which this invention is applicable is in commercial production. It consists of a metallic ball free to move in a sealed tube containing contacts that are bridged by the ball when it moves to one end of the tube. Air flow around the ball causes a pressure differential. The pressure differential causes a force proportional to and opposite the relative velocity of the ball with respect to the tube, A permanent magnet provides a bias force that retains the ball in a normal resting position away from the contacts and causes the velocity change required for switch closure to increase with the duration of the crash pulse. The proportionality of force to velocity makes the crash sensor an acceleration integrator that completes the firing circuit upon achievement of a predetermined velocity change. The variation of air viscosity with temperature is compensated for by making the tube and ball of particular and different stainless steels selected to have a difference in their thermal expansion coefficients such that the gap between the ball and tube changes with temperature as required to maintain the performance of the crash sensor over a wide temperature range. An elastomeric seal allows the tube to move in a plane perpendicular to the axis of the tube to prevent transmission of cross axis accelerations to the ball and the tube.
This crash sensor is expensive to manufacture and one reason for the cost is the high precision required of the ball and the tube. Another reason for the high cost for certain applications is that the tube must be made of material that is difficult to machine. Another reason for the high cost is the requirement in the present design that the electrical contacts must remain in good electrical contact with the ball as the ball moves over a distance of about one fourth of an inch. Further, in known designs, the contacts are connected with leadin wires and a diagnostic resistor by means such as welding or soldering. These processes create contamination during manufacturing that cannot be tolerated in the vicinity of the ball and tube thereby requiring additional components and processing steps to maintain isolation for cleanliness. Also, the thin material required for the contacts to have the required flexibility has the disadvantages of being difficult to handle and to connect with leadin wires of heavier material. Further, the diagnostic resistor must be a good quality resistor of relatively high precision which is not inexpensive.
The performance of this crash sensor depends upon flow of air between the ball and the inner diameter of the tube and is subject to a degree of variability depending on whether the ball moves near the center of the tube or near the inner wall. During a crash the venturi effect creates aerodynamic forces that urge the ball toward the center of the tube whereas lateral accelerations urge the ball toward a wall. When the ball moves near the wall there is a crescent shaped air channel between the ball and the tube. When the ball moves near the center of the tube there is an annular shaped air channel between the ball and the tube. The crescent shaped opening has about one half of the resistance to air flow as the annular opening. Therefore, the velocity change required for the ball to bridge the contacts varies substantially depending on the path of the ball.
Other crash sensors of similar design use a spring, typically one of the contacts, to provide the bias force that retains the ball in its home position. The permanent magnet bias seems to be preferable because of its simplicity but the two designs can be used interchangeably.
A safing sensor is a second crash sensor typically located on the firewall or elsewhere in the interior of the vehicle and is wired in series with one or more crash sensors located near the front of the vehicle. It prevents deployment of occupant protection systems in the event of sharp blows to a forward crash sensor which would not warrant deployment of the system.
Diagnostic systems typically include a diagnostic resistor that supplies a small current to the firing circuit during normal operation and means for monitoring that small current. Absence of the current indicates an open firing circuit. It is also desirable for the diagnostic system to detect small variations in the resistance of the firing circuit such as might be caused by corrosion of connectors. Circuits to accomplish this require high precision and current systems do not perform this function as well as is desired.
A circuit for measuring resistance is disclosed by the present inventor in U.S. Pat. No. 5,115,188 granted May 19, 1992 and titled Resistance Sensor And Switch.
A crash sensor having armature movement that is viscously damped by air viscosity and in which compensation for variation of air viscosity with temperature is accomplished by varying the normal resting position of the armature is disclosed by the present inventor in copending application Ser. No. 166,044 filed Mar. 9, 1988 and titled Mechanical Crash Sensing Switch With Latching In The Closed Position, now abandoned.
Integrated electronic circuits commonly include on a single chip of silicon both switching power transistors for controlling substantial power and circuitry for making the decisions as to when the switching power transistors should be conducting and not conducting. This circuitry may include voltage comparators, timing devices, and circuits for performing boolean logic.
Connections to semiconductor devices including integrated circuits are commonly made by attaching fine wires of materials such as gold or aluminum to pads on the semiconductor formed for that purpose. The pads are areas of the semiconductor chip that are sufficiently large for attachment to be made reliably by methods such as acoustic welding. The pads and the connections between the pads and the circuits on the semiconductor chip are made as part of the process of making the circuitry on the semiconductor chip.
Completed semiconductor chips are commonly mounted on a substrate which may be metal, ceramic, plastic, or other suitable material. The substrate typically includes connector pins for connecting the completed semiconductor device to other equipment. The pads on the chip are commonly connected to the connector pins by stitch bonding. The delicate chip and wiring of the electrically complete device are protected by encapsulation. Many encapsulation methods are known and several have been highly developed to meet the needs of the semiconductor industry. One method is to cover the completed chip and wiring with a small can and solder the can to the substrate which forms a hermetic seal. Another is to place the completed chip and wiring in a mold and flood the mold with uncured thermosetting plastic which is cured to form the required encapsulation.
Circuits for driving MOSFET power transistors are described in the literature. A review of such circuits is provided by chapter 6 of the second edition of POWER MOSFET TRANSISTOR DATA copyright 1986 and published by Motorola Inc.
A general object of this invention is to provide a crash sensor and diagnostic system for automotive vehicles which overcomes certain disadvantages of the prior art.